DESCRIPTION

The
pmdaCache
family of routines provide services to support the maintenance of
complex instance domains for Performance Co-Pilot PMDAs.
There is potentially one cache of information for each instance
domain, and for each instance the cache maintains:

-

external instance name (supplied by the PMDA)

-

internal instance identifier (assigned by
pmdaCacheStore)

-

state, where
active
instances are visible and part of the current
instance domain, and
inactive
instances are hidden, but not forgotten;
pmdaCacheStore
may be used to change the state of an instance

-

an opaque pointer to data that is associated with the instance,
but maintained by the PMDA

-

the last time the cache was saved and the instance had been marked as
active
at some point since the previous cache load or save operation

The semantics of a PCP instance domain require a number of rules to
be followed, namely:

1.

Each internal instance identifier must be unique and in the range
0 to 2^31 - 1.
This rule is enforced by the
pmdaCache
family of routines.

2.

The external instance name must be unique. When the instance name
contains a space, it is further constrained such that the name to
the left of the first space (the short name) must also be unique.
Refer to the INSTANCE NAME MATCHING section below.
The PMDA must honor this rule, the
pmdaCache
family of routines will detect attempts to violate this rule.

3.

Where an external instance name corresponds to some object or entity,
there is an expectation that the association between the name and
the object is fixed, e.g. ``/dev/hda'' is always the name of the same disk
on a particular system.
This rule is perhaps the responsibility of the PMDA, but is often
a characteristic of the environment in which the PMDA runs.

4.

It is preferable, although not mandatory, for the association between
and external instance name and an internal instance identifier to
be persistent.
This rule is supported by the
pmdaCache
family of routines.

The visible interface to the cache is oriented towards the PMDA
developer who is most concerned about the names of instances, while
the details of how the rest of the PCP infrastructure
expects the internal instance identifiers
to be managed is not relevant.

Instances are updated in the cache for instance domain
indom
by calling
pmdaCacheStore
with the external name of the instance passed via
name.
The opaque pointer
private
may be used to associate additional data with the entry in the cache;
if no such data is required,
private
should be NULL.
Any manipulation of the additional data (including allocation or
freeing) is the responsibility of the PMDA caller, as the cache simply
maintains the pointer to the data
(passed via
private).

The
flags
argument controls how the instance should be processed in the cache
as follows:

PMDA_CACHE_ADD

Insert the entry into the cache if it is not already there and mark
it
active.
If the entry is already in the cache mark it
active.

PMDA_CACHE_HIDE

Mark the entry in the cache as
inactive,
but remember the
details of the association between the
external instance name and the internal instance identifier.
Entries that are
inactive
will be hidden from cache traversal via PMDA_CACHE_WALK_NEXT
operations, but remain visible to
pmdaCacheLookup
and
pmdaCacheLookupName
requests.

PMDA_CACHE_CULL

Remove the entry from the cache.

On success
pmdaCacheStore
will return the internal instance identifier of the associated cache
entry.
Valid instance identifiers are guaranteed to be unique and non-negative.
Failure will be indicated by a negative value (suitable for decoding
with
pmErrStr(3))
and most likely PM_ERR_INST to indicate the requested instance is not
in the cache, or -EINVAL to indicate a potential violation of the
short name uniqueness property
(see the INSTANCE NAME MATCHING section below).

pmdaCacheLookup
is used to search the
entries in the cache based on the internal
instance identifier
inst.

On success the return value will be PMDA_CACHE_ACTIVE or PMDA_CACHE_INACTIVE
(depending on the
active
or
inactive
state of the cache entry),
name
(if not NULL) and
private
(if not NULL)
will be set to the external instance name and the associate additional data
area as provided when the instance was last activated via
pmdaCacheStore.

pmdaCacheLookup
failure is indicated by a negative return value
suitable for decoding with
pmErrStr(3).

The
pmdaCacheLookup
interface is required by the PMDA's fetch callback
that is registered via
pmdaSetFetchCallback(3).
Here the internal instance identifier is passed to the fetch callback
to identifier for which instance a value is required.
Typical usage is shown in the code fragment below.

pmdaCacheLookupName
is used to search the
entries in the cache based on the external
instance name
name.

On success the return value will be PMDA_CACHE_ACTIVE or PMDA_CACHE_INACTIVE
(depending on the
active
or
inactive
state of the cache entry),
inst
(if not NULL) and
private
(if not NULL)
will be set to the internal instance identifier and the associate additional data
area as provided when the instance was last activated via
pmdaCacheStore.

pmdaCacheLookupName
failure is indicated by a negative return value
suitable for decoding with
pmErrStr(3).

The
pmdaCacheLookupName
interface is useful for PMDAs wishing to update an instance domain based
on the external instance names.

To avoid a persistent cache growing without bound,
pmdaCachePurge
can be used to cull all entries that have
not
been
active
in the last
recent
seconds.
For performance reasons, the time accounting is imprecise and the entries
are timestamped
at the time of the next cache save operation
after
the entry has been added or marked
active
(refer to PMDA_CACHE_SAVE and PMDA_CACHE_SYNC below).
On success
pmdaCachePurge
returns the number of culled entries, else in the case of an error
the return value is negative (and suitable for decoding with
pmErrStr(3)).

pmdaCacheOp
may be used to perform additional operations on the cache as follows:

PMDA_CACHE_LOAD

The cache can optionally be maintained as a persistent external file,
so that the mapping of instance names to instance identifiers is persistent
across executions of a PMDA.
This operation loads the cache from the external file, and then
all new cache entries are marked
inactive,
and the additional
data pointer is set to NULL.
Entries loaded from the external file are checked against the current
cache contents and if the instance name and instance identifiers match
then the state in the cache (active
or
inactive)
is not changed. Should a mismatch be found (same instance name and
different instance identifier, or same instance identifier and different
instance name) then the entry from the external file is ignored
and a warning is issued on
stderr.
Typically a PMDA would only
perform this operation once per execution.

PMDA_CACHE_SAVE

If any instance has been added to, or deleted from, the instance
domain since the last PMDA_CACHE_LOAD, PMDA_CACHE_SAVE or PMDA_CACHE_SYNC
operation, the
entire
cache is written to the external file as a bulk operation.
This operation is provided for PMDAs that are
not
interested
in using
pmdaCachePurge
and simply want the external file to reflect the set of known instances
without accurate details of when they were last marked
active.

Returns the number of instances saved to the external file, else 0
if the external file was already up to date.

PMDA_CACHE_SYNC

Within an instance domain,
if any instance has been added to, or deleted from, or marked
active
since the last PMDA_CACHE_LOAD, PMDA_CACHE_SAVE or PMDA_CACHE_SYNC
operation, the
entire
cache is written to the external file as a bulk operation.
This operation is similar to PMDA_CACHE_SAVE, but will save the
instance domain more frequently so the timestamps more
accurately match the semantics expected by
pmdaCachePurge.

Returns the number of instances saved to the external file, else 0
if the external file was already synchronized.

PMDA_CACHE_CHECK

Returns 1 if a cache exists for the specified instance domain,
else 0.

PMDA_CACHE_REUSE

When a new instance is added to the cache,
the default strategy is to assign instance identifiers in a monotonic
increasing
manner. Once the maximum possible instance identifier value has been
assigned, the strategy changes to one where starting from 0,
the next available unused instance identifier will be used.
Calling
pmdaCacheOp
with PMDA_CACHE_REUSE forces an irreversible change to the second
(reuse) strategy. This may be useful in cases where there is a
desire to restrict the allocated instance identifiers to smaller
values. The prevailing strategy will be saved and restored across
PMDA_CACHE_SAVE and PMDA_CACHE_LOAD operations.

PMDA_CACHE_REORG

Reorganize the cache to allow faster retrieval of
active
entries, and the cost of slower retrieval for
inactive
entries, and reclaim any culled entries. The cache may be internally
re-organized as entries are added, so this operation is not required
for most PMDAs.

PMDA_CACHE_WALK_REWIND

Prepares for a traversal of the cache in ascending instance identifier
sequence.

PMDA_CACHE_WALK_NEXT

Fetch the next
active
instance identifier from the cache. Requires a prior
call using PMDA_CACHE_WALK_REWIND and will return -1 when all instances
have been processed.

Only one cache walk can be active at any given time, nesting calls
to PMDA_CACHE_WALK and PMDA_CACHE_REWIND will interfere with each
other.

PMDA_CACHE_ACTIVE

Changes
everyinactive
entry in the cache to be marked
active.

PMDA_CACHE_INACTIVE

Changes
everyactive
entry in the cache to be marked
inactive.

PMDA_CACHE_CULL

Remove
every
entry from the cache.

PMDA_CACHE_SIZE

Return the number of entries in the cache (includes
active,
inactive
and any culled entries that have not yet been reclaimed).

PMDA_CACHE_SIZE_ACTIVE

Return the number of
active
entries in the cache.

PMDA_CACHE_SIZE_INACTIVE

Return the number of
inactive
entries in the cache.

PMDA_CACHE_DUMP

Dump the current state of the cache on
stderr.

pmdaCacheOp
returns a non-negative value on success, and failure is indicated
by a negative return value (suitable for decoding
with
pmErrStr(3)).

OTHER CONSIDERATIONS

When the
pmdaCache
routines are used for particular instance domain,
pmdaInstance (3)
and the instance domain enumeration behind
pmdaFetch(3)
will attempt to extract instance domain information from the cache, thereby avoiding
reference to the
pmdaIndom
data structures that have historically been used to define instance domains
and service instance requests.
A PMDA can adopt a hybrid approach and choose to implement some instance
domains via the traditional
pmdaIndom
method, and others via the
pmdaCache
approach, however attempts to manage the
same
instance domain by both
methods will result in the
pmdaCache
method silently prevailing.

If
all
instances in a PMDA are to be serviced from a
pmdaCache
then a
pmdaIndom
is not required, and the
pmdaInit (3)
call becomes

pmdaInit(dp, NULL, 0, metrictab, nmetrics);

However, the PMDA will need to explicitly initialize the
indom
field of the
pmDesc
in the
metrictab
entries, as this cannot be done by
pmdaInit(3)
if
indomtab
is missing entries for the instance domains maintained in the cache.

Independent of how the instance domain is being maintained,
to refresh an instance domain prior to a fetch or an instance domain
operation, the standard methods of a ``wrapper'' to the
pmdaInstance (3)
and
pmdaFetch (3)
methods should be used.

Refer to the
simple
PMDA source code for an example use of the
pmdaCache
routines.

INSTANCE NAME MATCHING

The following table summarizes the ``short name'' matching semantics
for an instance domain.

name in

pmdaCacheLookup

result

cache

name

foodle

foo

no match (PM_ERR_INST)

foo

foodle

no match (PM_ERR_INST)

foo

foo

match

foo bar

foo

match on short name (instance identifier)

foo bar

foo bar

match on full name (instance identifier)

foo

foo bar

bad match (-EDOM)

foo bar

foo blah

bad match (-EDOM)

FILES

Cache persistence uses files with names constructed from the
indom
within the
$PCP_VAR_DIR/config/pmda
directory.